Field of the Invention
The invention relates to an electric discharge machining apparatus that supplies a current pulse to a machining gap formed between a work piece and a tool electrode to machine the work piece. In particular, the invention relates to an electric discharge machining apparatus that is capable of supplying a current pulse with a first polarity, in which the work piece is a positive potential and the tool electrode is a negative potential, and supplying a current pulse with a second polarity, in which the work piece is the negative potential and the tool electrode is the positive potential.
Description of Related Art
A polarity, in which the work piece is the positive potential and the tool electrode is the negative potential, is called “straight polarity”. A polarity, in which the work piece is the negative potential and the tool electrode is the positive potential, is called “reverse polarity”. The polarity of a power pulse is an important condition for electric discharge machining. Generally, in order to prevent electric corrosion of the work piece, a wire electric discharge machine maintains the average of voltages in the machining gap (“gap voltage”) at 0V as much as possible by switching the polarity.
The wire electric discharge machine that applies a high frequency AC voltage to the machining gap to machine the work piece is widely known. With such a wire electric discharge machine, the current pulse having a short ON time improves the surface roughness. However, as the size of the machining gap and the machining area change, the impedance in the machining gap changes. If the impedance changes significantly, the no-load voltage does not go up high enough and the power supplied to the machining gap would be lower than expected.
Patent Literature 1 discloses an electric discharge machining apparatus, in which an impedance matching circuit is provided between an AC power supply and the machining gap. The AC power supply is connected to the machining gap through a proper power cable. The impedance matching circuit suppresses undesirable influence of the electrostatic capacity that exists in the power cable.